The impact of the El Niño 1997-1998 phenomenom on plankton dynamics was studied during 1997 at the Punta Morales estuary, Gulf of Nicoya, Pacific coast of Costa Rica.The study covered dry season/transition and the rainy season. Phytoplankton (microphytoplankton > 30μm and nanophytoplankton) were collected at two depths (50 and 10% light incidence) using a 5 L Niskin bottle, and samples taken to determine chlorophyll a. Temperature, salinity, oxygen, and Secchi depth were measured. Horizontal sub-surface zooplankton hauls were conducted with a conic zooplankton net of 0.49 m diameter and 280μm mesh width, supplied with a flowmeter. Surface sea water temperature average was 29.9 ± 0.9°C, with a maximum of 31.5°C in April and a minimun of 28°C in March and October. Chlorophyll a concentration (phytoplankton net) averaged 3.1± 1.7 mg/m3, with higher values during the rainy season and lower values during the transition. Nanophytoplankton chlorophyll a concentration averaged 2.5±1.2 mg/m3, with a maximum during the transition season. For both fractions there were significant differences between transition and rainy seasons, and for nanophytoplankton between dry and transition seasons (p < 0.05). In the last case, differences were explained by temperature. Herbivorous copepods dominated the abundance and biomass of zooplankton, with a biomass maximun of 167.3 in October and a minimun of 7.1 mg DW/m3 in December. These values are higher than those found some years ago in the same zone and those reported for some places in the Caribbean. The El Niño 1997-1998 phenomenon in the plankton dynamics appears to have produced a change in the phytoplankton structure. This is the first attempt to evaluate the possible impact of El Niño on the plankton dynamics of the Pacific coast of Central America.
Keywords: Plankton, Pacific, El Niño, Costa Rica, copepods